Method of manufacturing oxide superconducting materials

ABSTRACT

Bi-based oxide superconducting films are deposited on Bi-based oxide ceramic substrates by screen printing, laser sputtering and other coating methods wherein the substrates have a similar crystalline structure as the Bi-based oxide superconducting films.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing oxidesuperconductor elements, and more particularly to a method ofmanufacturing Bi-based superconducting ceramic materials.

Recently, superconducting ceramics comprising Bi have been discovered aspromising materials which can be formed to have high Tc. The Bi-basedsuperconducting material takes two phases, in one of which, called thelow Tc phase, the critical temperature reaches up to 80° K., and in theother, called the high Tc phase, the critical temperature reaches up to110° K. An example of the stoichiometric formula of the low Tc ceramicis Bi₂ Sr₂ CaCu₂ O_(8-x) having its c-axis length of 30 Å. An example ofthe stoichiometric formula of the high Tc ceramic is Bi₂ Sr₂ Ca₂ Cu₃O_(10-x) having its c-axis length of 36 Å. In this low Tc phase, thereare formed three CuO planes within one unit structure thereof. Inaccordance with recent reports, there are other phases having theirc-axes of 42 Å and 48 Å which phases there are formed CuO-(Sr, Ca)planes in addition to the three CuO plane. It is believed for thisreason that the resistivity of Bi-based superconductors begins to falldown at 150° K. or higher temperatures.

In spite of past efforts, it has been still difficult to increase theproportion of the high TC phase in the formation of Bi-basedsuperconducting ceramics. Also, single crystals of the high Tc phasehave not be formed yet.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof producing high Tc superconducting ceramic materials.

In order to accomplish the above and other objects and advantages, seedcrystals or substrate are prepared from other materials which can beeasily formed to have molecular structures similar as superconductingceramics. For example, Bi-based layered dielectrics expressed by thegeneral formula, (Bi₂ O₂) (A_(m-1) B_(m) O_(3m+1)) where m>1, tend tohave very similar structure as superconducting ceramics. The a-axislengths and the b-axis lengths thereof are almost equal to those ofBi-based superconducting ceramics. In this formula, A stands for analkali earth metal such as Ca. B stands for a rear earth metal such asTi. These oxides are not superconducting ceramics while the formation iseasier than that of superconducting ceramics. Among these are Bi₂(Sr_(1-x) La_(x))₄ Cu₃ O_(y) (x=0.05-0.4, y=8-12; c-axis length is 32.8Å), Bi₄ Ti₃ O₁₂ (c-axis length is 32.8 Å), CaBi₄ Ti₄ O₁₅ (c-axis lengthis 41-42 Å), SrBi₄ Ti₄ O₁₅ (c-axis length is 41-42 Å), BaBi₄ Ti₄ O₁₅(c-axis length is 41-42 Å), PbBi₄ Ti₄ O₁₅ (c-axis length is 41-42 Å),Sr₂ Bi₄ Ti₅ O₁₈ (c-axis length is 48.8 Å), Ba₂ Bi₄ Ti₄ O₁₈ (c-axislength is 50.3 Å), Pb₂ Bi₄ Ti₅ O₁₈ (c-axis length is 49.7 Å), When asuperconducting ceramic material is desired to be formed, a suitableceramic material (called seed ceramic material) is first selected, e.g.among from the above material to have its c-axis length close to that ofthe superconducting ceramic material to be formed thereon. On the otherhand, superconducting Bi₂ (Sr, Ca)₆ Cu₅ O_(14+x) has its c-axis lengthof 48 Å. According to this method, the high Tc phase can be selectivelyformed on the seed ceramic.

Alternatively, the seed ceramic material can be selected to have itsc-axis length which constitutes a simple ratio, such as n/m (n and m areintegers), to the c-axis length of the superconducting ceramic materialto be formed. For example, it was observed that the high Tc phase of 36Å c-axis length was selectively grown by virtue of the existence of asemiconductor phase of 24 Å c-axis length. Namely, 24:36=2:3. Ceramicswhose c-axis lengths constitute simple ratio to those of superconductingceramics are, for example, Bi₃ TiNbO₉ (c-axis length is 25 Å), Bi₃TiTaO₉ (c-axis length is 25 Å), Bi₃ Ti₄ NbO₉ (c-axis length is 25 Å),CaBi₂ Nb₂ O₉ (c-axis length is 25 Å), Bi₃ TiTaO₉ (c-axis length is 25Å), CaBi₂ Ta₂ O₉ (c-axis length is 25 Å), SrBi₂ Nb₂ O₉ (c-axis length is25 Å), SrBi₂ Ta₂ O₉ (c-axis length is 25 Å), Ba₂ Bi₂ Nb₂ O₉ (c-axislength is 25 Å), CaSr₂ Bi₄ Ti₅ O₁₈ (c-axis length is 49-50 Å), Sr₂ Bi₄Ti₅ O₁₈ (c-axis length is 49-50 Å), Ba₂ Bi₄ Ti₅ O₁₈ (c-axis length is49-50 Å), Pb₂ Bi₄ Ti₅ O₁₈ (c-axis length is 49-50 Å).

Furthermore, the inventor has found new compound oxide materials, whichcan be represented generally by the formula Bi₂ LaCa_(n) Cu_(n) O_(y)(n=1, 2 or 3; y=8-12, called BLCCO hereafter) and have same molecularstructures as Bi-based superconducting ceramics. The seed crystals ofthis type could be effectively used for forming single crystals of thesuperconducting ceramics. The a-axis length and the b-axis length of theseed ceramics are same as the Bi-based superconducting ceramics. Thec-axis length is 24 Å (n=1), 30 Å (n=2) and 36 Å (n=3), which are equalto the corresponding figures of superconducting ceramics in the form ofBi₂ Sr₂ Ca_(n-1) Cu_(n) O_(y) (n=1, 2 or 3, y=8-12). With this seedcrystals, single crystalline BLCCO can be formed on the order of 2 cm.

BRIEF DESCRIPTION OF THE DRAWING

This invention can be better understood from the following detaileddescription when read in conjunction with the drawing in which

FIG. 1 is a graphical diagram showing the resistivity of asuperconducting ceramic with changing temperature in accordance with thepresent invention.

FIG. 2 is a graphical diagram showing the resistivity of asuperconducting ceramic with changing temperature in accordance with thepresent invention.

FIG. 3 is a graphical diagram showing the magnetization of asuperconducting ceramic with changing temperature in accordance with thepresent invention.

FIG. 4 is a graphical diagram showing the resistivity of asuperconducting ceramic with changing temperature in accordance with thepresent invention.

FIG. 5 is a shematic diagram showing a crystal pulling apparatus.

FIG. 6 is a graphical diagram showing the resistivity of asuperconducting ceramic with changing temperature in accordance with thepresent invention.

FIG. 7 is a graphical diagram showing the resistivity of asuperconducting ceramic with changing temperature in accordance with thepresent invention.

FIG. 8 is a schematic diagram showing an laser sputtering apparatus foruse in an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, several experiments are illustrated in accordance withembodiments of the present invention making use of the screen pressprinting.

EXPERIMENT 1

Bi₂ O₃ and TiO₂ powders of 99.9% purity were prepared and mixed witheach other. After firing at 800° C. for 12 hours in air, the mixture wascompacted into a circular tablet of 1 mm thickness and 20 mm diameter.The tablet was fired again at 1000° C. for 12 hours in air. It wasconfirmed by means of x-ray diffraction that the stoichiometriccomposition of the tablet was in the single phase of Bi₄ Ti₃ O₁₂. Thec-axis length of Bi₄ Ti₃ O₁₂ is 32.8 Å.

Next, Bi₂ O₃, SrCO₃, CaCO₃ and CuO in prescribed amount were prepared inpowder form (99.9% purity) and mixed with each other so thatBi:Sr:Ca:Cu=2:2:2:3. The mixture was fired at 800° C. for 12 hours inair and ground into powders of 1-5 micrometers average diameter. Thepowders were mixed with ethanol and applied to the surface of the Bi₄Ti₃ O₁₂ tablet in film form by screen printing. The film was fired at900° C. for 30 min and gradually cooled at 60° C./hour to roomtemperature. The thickness of the film was measured to be 30micrometers. The c-axis was measured to be 36 Å. FIG. 1 shows theresistivity of the film versus the temperature. As shown in the figure,the critical temperature was 110° K. In accordance with the X-raydiffraction, the film was proved to be in the high Tc phase of Bi₂ Sr₂Ca₃ O_(10+x). The critical superconducting current density was measuredto be 50,000 Å/cm².

EXPERIMENT 2

SiCO₃, Bi₂ O₃ and TiO₂ powders of 99.9% purity were prepared and mixedwith each other. After firing at 800° C. for 12 hours in air, themixture was compacted into a circular tablet of 1 mm thickness and 20 mmdiameter. The tablet was fired again at 1000° C. for 12 hours in air. Itwas confirmed by means of x-ray diffraction that the stoichiometriccomposition of the tablet was Sr₂ Bi₄ Ti₅ O₁₈ including a few amount ofSr₂ Bi₄ Ti₄ O₁₅. The c-axis length of Sr₂ Bi₄ Ti₅ O₁₈ is 49-50 Å.

Next, Bi₂ O₃, SrCO₃, CaCO₃ and CuO in prescribed amount were prepared inpowder form (99.9% purity) and mixed with each other so thatBi:Sr:Ca:Cu=2:3:3:5. The mixture was fired at 800° C. for 12 hours inair and ground into powders of 1-5 micrometers average diameter. Thepowders were mixed with ethanol and applied to the surface of the Sr₂Bi₄ Ti₅ O₁₈ tablet in film form by screen printing. The film was firedat 900° C. for 30 min and gradually cooled at 60° C./hour to roomtemperature. The thickness of the film was measured to be 30micrometers. The c-axis was measured to be not shorter than 40 Å. FIG. 2shows the resistivity of the film versus the temperature. As shown inthe figure, the critical temperatuer was 110° K. In accordance with theX-ray diffraction, the film was proved to be in the high Tc phase of(Bi₂ O₂)(Sr₃ Ca.sub. 3 Cu₅ O_(18+x)). Meisner effect was confirmed bymeasuring magnetization using a SQUID. Namely, as illustrated in FIG. 3,the magnetization became negative when cooled below about 150° K. Theproportion of the high Tc phase (Tc≈150° K.) was estimated by themagnetization at 3 to 8%. The critical superconducting current densitywas measured to be 50,000 Å/cm².

EXPERIMENT 3

Nb₂ O₅, Bi₂ O₃ and TiO₂ powders of 99.9% purity were prepared and mixedwith each other. After firing at 800° C. for 12 hours in air, themixture was compacted into a circular tablet of 1 mm thickness and 20 mmdiameter. The tablet was fired again at 1000° C. for 12 hours in air. Itwas confirmed by means of x-ray diffraction that the stoichiometriccomposition of the tablet was in the single phase of Bi₃ TiNbO₉. Thec-axis length of Bi₃ TiNbO₉ is 25.1 Å.

Next, Bi₂ O₃, SrCO₃, CaCO₃ and CuO in prescribed amount were prepared inpowder form (99.9% purity) and mixed with each other so thatBi:Sr:Ca:Cu=2:2:2:3. The mixture was fired at 800° C. for 12 hours inair and ground into powders of 1-5 micrometers average diameter. Thepowders were mixed with ethanol and applied to the surface of the Bi₃TiNbO₉ tablet in film form by screen printing. The film was fired at900° C. for 30 min and gradually cooled at 60° C./hour to roomtemperature. The thickness of the film was measured to be 30micrometers. The c-axis was measured to be 36 Å. FIG. 4 shows theresistivity of the film versus the temperature. As shown in the figure,the critical temperatuer was 110° K. In accordance with the X-raydiffraction, the film was proved to be in the high Tc phase of Bi₂ Sr₂Ca₃ O_(10+x). The critical superconducting current density was measuredto be 50,000 Å/cm².

The superconducting ceramic can be formed in accordance with the presentinvention by the crytal pulling technique as described in thefollowings.

EXPERIMENT 4

Bi₂ O₃ and TiO₂ powders of 99.9% purity were prepared and mixed witheach other. The mixture was kept in a crucible made of alumina at 1200°C. for seven days, and then crushed, from which crushed material a seedcrystal of Bi₄ Ti₃ O₁₂ was selected and taken up. The size of thecrystal was of the order of about 3 mm×3 mm×1 mm.

Next, Bi₂ O₃, SrCO₃, CaCO₃ and CuO in prescribed amount were prepared inpowder form (99.9% purity) and mixed with each other so thatBi:Sr:Ca:Cu=4:2:2:3. The mixture was fired at 800° C. for 12 hours inair and molten in a crucial at 890° C. A superconducting crystal wasproduced from the fired mixture by the crystal pulling technique.

FIG. 5 illustrates the furnace construction which was used for thecrystal growth in accordance with the present invention. The ceramicmixture 2 was placed in a crucible 1 seated on a ceramic base. Athermocouple is embeded in the base in order to measure the temperatureof the crucible 1. A quartz tube was provided surrounding the base andthe crucible. A r.f. coil 6 outside the tube then completes theassembly.

The charge in the crucible 1 was molten at 890° C. The seed crystal wassupported by a rod 4 so that its c-axis was oriented parallel with thesurface of the molten ceramic. Then the seed crystal was pulled up at 5mm/hour. As a result, a single crystalline Bi-based superconductingceramic as large as 5 mm×30 mm×0.5 mm was obtained. FIG. 6 shows theresistivity of the film versus the temperature. As shown in the figure,the critical temperatuer was 110° K. The critical superconductingcurrent density was measured to be 50,000 Å/cm².

EXPERIMENT 5

Bi₂ O₃, La₂ O₃, CaCO₃ and CuO powders of 99.9% purity were prepared andmixed with each other so that Bi:La:Ca:Cu=2:1:2:1. The mixture was keptin a crucible made of alumina at 1000° C. for an hour, followed bygradual cooling at 10° C./hour, and then crushed, from which crushedmaterial a seed crystal of Bi₂ LaCa₂ Cu₂ O_(y) was selected and takenup. The size of the crystal was about 3 mm×3 mm×1 mm.

Next, Bi₂ O₃, SrCO₃, CaCO₃ and CuO in prescribed amount were prepared inpowder form (99.9% purity) and mixed with each other so thatBi:Sr:Ca:Cu=1:1:1:3. The mixture was fired at 800° C. for 12 hours inair and molten in a crucible at 890° C. A superconducting crystal wasproduced from the molten raw material by the crystal pulling technique.

The charge in the crucible was molten at 890° C. The seed crystal wassupported by a rod 4 so that its c-axis was adjusted parallel with thesurface of the molten ceramic. Then the seed crystal was pulled up at 5mm/hour. As a result, a single crystalline Bi-based superconductingceramic as large as 20 mm×20 mm×0.5 mm was obtained. FIG. 7 shows theresistivity of the film versus the temperature. As shown in the figure,the critical temperatuer was 82° K. The critical superconducting currentdensity was measured to be 10,000 Å/cm².

EXPERIMENT 6

The ceramics represented by Bi₂ (Sr_(1-x) La_(x))₄ Cu₃ O_(y)(x=0.05-0.4, y=8-12; c-axis length is 32.8 Å) have the same structure asthe high Tc phase of the Bi-based superconducting ceramics. The a-axisand b-axis lengths are 5.4 Å and 3.6 Å, which are equal to thecorresponding figures of the Bi-based superconducting ceramics. Thesematerials, however, can not exhibit superconductivity since the carrierdensity therein is too low.

The Bi₂ (Sr_(1-x) La_(x))₄ Cu₃ O_(y) ceramic was prepared by flux methodwith CuO as a melting agent. First, Bi₂ O₃, SrCO₃, La₂ O₃, and CuOpowders of 99.9% purity were prepared and mixed with each other so thatBi:Sr:La:Cu=2:3.5:0.5:3. The mixture was placed in a crucible and firedat 1000° C. for 5 hours followed by gradual cooling to 800° C. at 10°C./hour. Then, the cooled charge was crushed in order to pick up asuitable seed crystal therefrom.

Next, Bi₂ O₃, SrCO₃, CaCO₃ and CuO in prescribed amount were prepared inpowder form (99.9% purity) and mixed with each other so thatBi:Sr:Ca:Cu= 4:2:2:3. The mixture was fired at 800° C. for 12 hours inair and molten in a crucible at 890° C. A superconducting crystal wasproduced from the molten material by the crystal pulling technique.

The charge in the crucible was molten at 890° C. The seed crystal wassupported by a rod 4 so that its c-axis was adjusted parallel with thesurface of the molten ceramic. Then the seed crystal was pulled up at 5mm/hour. As a result, a single crystalline Bi-based superconductingceramic of 5 mm×50 mm×0.5 mm was obtained. The critical temperatuer wasmeasured to be 106° K. The critical supercoducting current density wasmeasured to be 50,000 Å/cm².

The superconducting ceramic can be formed in accordance with the presentinvention by the laser sputtering technique as described in thefollowings.

EXPERIMENT 7

Bi₂ O₃, SrCO₃, La₂ O₃, and CuO powders of 99.9% purity were prepared andmixed with each other so that Bi:Sr:La:Cu=2:3.5:0.5:3. The mixture wascompressed into a tablet and fired at 850° C. for 24 hours. Then asubstrate of Bi₂ (Sr_(1-x) La_(x))₄ Cu₃ O_(y) was obtained. Thesubstrate was disposed in a laser sputtering apparatus illustrated inFIG. 8. The apparatus comprises a vacuum chamber 16, a gas feedingsystem 21, an exhaustion system comprising a turbo molecular pump 19 anda rotary pump 20, a substrate holder provided with a heater 14 therein,an eximer laser 11 and an associated lens 12 which focuses the laserrays emitted from the laser 11 throught a light window 3 to a target 17appropriately positioned in the chamber. The target 17 was prepared froma Bi-based superconductor of Bi₂ Sr₂ Ca₂ Cu₃ O_(y) in accordance withthe foregoing description.

After introducing argon and oxygen at Ar:O₂ =4:1 and maintaining thewhole pressure at 0.1 Torr, the target 17 was slowly disintegrated dueto bombardment by laser rays and the material of the target wasdeposited on the substrate 15 of Bi₂ (Sr_(1-x) La_(x))₄ Cu₃ O_(y). Thesubstrate temperature was adjusted at 650° C. by means of the heater 14.The deposition speed was about 10 nm/min. The thickness of the depositedfilm was 500 nm. Through the measurement of resistivity, Tc was measuredto be 106° K. This high Tc was attributed to the high Tc phase.

EXPERIMENT 8

The ceramics represented by Bi₂ Sr₄ Fe₃ O_(y) (y=8-12) have the samesturcture as the high Tc phase of the Bi-based superconducting ceramics.The a-axis and b-axis lengths are 5.4 Å and 3.6 Å, which are equal tothe corresponding figures of the Bi-based superconducting ceramics. Inthis ceramic structure, Fe and Sr atoms replace Cu and Ca sites of theBi-based superconducting ceramics.

Bi₂ O₃, SrCO₃, La₂ O₃, and CuO powders of 99.9% purity were prepared andmixed with each other so that Bi:Sr:La:Cu=2:3.5:0.5:3. The mixture wascompressed into a tablet and fired at 850° C. for 24 hours. Then asubstrate of Bi₂ Sr₄ Fe₃ O_(y) was obtained. This substrate was disposedin a laser sputtering apparatus illustrated in FIG. 8. The target 17 wasprepared from a Bi-based superconductor (Bi₂ Sr₂ Ca₂ Cu₃ O_(y)) inaccordance with the foregoing description.

After introducing argon and oxygen at Ar:O₂ =4:1 and maintaining thewhole pressure at 0.1 Torr, the target 17 was slowly disintegrated dueto bombardment by laser rays and the material of the target wasdeposited on the substrate 15 of Bi₂ Sr₄ Fe₃ O_(y). The substratetemperature was adjusted at 650° C. by means of the heater. Through themeasurement of resistivity, Tc was measured to be 106° K. This Tc wasattributed to the high Tc phase.

While several embodiments have been specifically described by way ofexamples, it is to be appreciated that the present invention is notlimited to the particular examples described and that modifications andvariations can be made without departing from the scope of the inventionas defined by the appended claims. Some examples are as follow.

Although three methods were employed for forming superconductor on thesubstrates in the foregoing embodiments, the superconducting film, thesuperconductor can be formed by other methods which have been used forforming superconducting ceramics such as vacuum evaporation, reactiveevaporation, chemical vapor reations, spray pyrolysis.

What is claimed is:
 1. A method of manufacturing crystalline Bi-basedsuperconducting ceramic comprising the steps of:preparing a substratemade of a non-superconducting Bi-based oxide ceramic which has a similarcrystalline structure as the superconducting ceramic to be manufacturedby this method; and forming the Bi-based superconducting ceramic on saidsubstrate by a method selected from the group consisting of evaporation,sputtering, screen printing, chemical vapor deposition and spraypyrolysis, wherein an a-axis length and a b-axis length of said Bi-basedoxide ceramic substrate are substantially equal to those of saidsuperconducting ceramic.
 2. The method of claim 1 wherein the c-axislength of said Bi-based oxide ceramic substrate is close to that of saidsuperconducting ceramic.
 3. The method of claim 1 wherein said formingstep is carried out by screen printing.
 4. The method of claim 1 whereinsaid forming step is carried out by laser sputtering.
 5. The method ofclaim 1 wherein said Bi-based oxide ceramic substrate is selected amongfrom the group consisting of Bi₂ (Sr_(1-x) La_(x))₄ Cu₃ O_(y), Bi₄ Ti₃O₁₂, CaBi₄ Ti₄ O₁₅, SrBi₄ Ti₄ O₁₅, BaBi₄ Ti₄ O₁₅, PbBi₄ Ti₄ O₁₅, Sr₂ Bi₄Ti₅ O₁₈ and Ba₂ Bi₄ Ti₄ O₁₈ Pb₂ Bi₄ Ti₅ O₁₈.
 6. A method of forming aBi-based superconducting ceramic comprising the steps of:preparing aBi-based substrate which is represented by the general formula (Bi₂ O₂)(A_(m-1) B_(m) O_(3m+1)) where A stands for an alkali earth metal, Bstands for a rare earth metal and m>1; and forming the Bi-basedsuperconducting ceramic on said substrate by a method selected from thegroup consisting of evaporation, sputtering, screen printing, chemicalvapor deposition and spray pyrolysis.